A recent effort in the technical community (for example, (1) a collection of papers presented at the AIAA 10th Communication Satellite Systems Conference, Orlando, Fla., March 1984 --and published under the same title by the AIAA, 1633 Broadway, New York, N.Y. 10019; (2) an article by A. Chiaviello entitled, "2.sup.o Satellite Spacing: An FCC Challenge to Microwave Designs", published in the September 1983 edition of the Microwave Journal; (3) an article by M. Kachmar entitled, "`Haves` and `have nots` to butt heads at Space WARC", published in the November 1984 edition of the Microwave & RF periodical to reduce the geostationary satellite orbit separation from about 3.5 to 4 degrees down to as low as 2 degrees will effectively increase the number of satellites, thereby substantially increasing the total channel capacity for satellite communications. One critical technical problem incident to this effort is the interference between adjacent satellites operating in the same frequency spectrum and the same polarization. This has long been considered the main obstacle in reducing the geostationary satellite spacing.
One common way of countering the adjacent satellite interference is to use an interference canceler. In addition to the ordinary receiving apparatus (called main antenna sometimes), an interference canceler usually consists of a secondary (called auxiliary sometimes) antenna or feed and a network to generate a replica of the interfering signal. This is then used to subtract the interfering signal received by the main receiving apparatus such that the interfering signal is substantially cancelled at the output of the canceler.
One of the problems associated with the aforesaid technique is that an adaptive network is needed to combat the time-varying nature of the interfering signal. In addition, because the antenna patterns are frequency dependent, a frequency dependent cancellation network is necessary to perform wideband interference cancellation. Also, because the auxiliary feed, or the antenna, will receive both the interference signal and the desired signal, some method has to be adopted to separate them in performing the interference cancellation.
A known method of solving this last problem calls for inserting pilot tones in the interfering signal, which can then be used to identify the interfering signal and obtain the tap weights for adaptive cancellation. See, for example, CCIR Report 875, "A Survey of Interference Cancelers for Application in the Fixed Satellite Service" CCIR Recommendation & Report, Volume IV --Part I, 1982. Although the interfering signal is canceled, the desired signal is also degraded somewhat as will be disclosed more clearly in the detailed description of the present invention. It is desirable to obtain a better method, and apparatus to implement the method, for canceling the interfering signal by avoiding the aforesaid problem of degradation of the desired signal and to simplify the method for separating the desired signals and the interfering signals.